Recently, semiconductor laser diodes have come to be widely used in a variety of types of electrical equipment, for example, laser printers, optical disk apparatuses, fiber-optic communication apparatuses, and mobile phones, because of their compact size, low cost, and ease of use.
However, the current/light-emission intensity characteristics of the semiconductor laser diode are dependent on temperature. Accordingly, it is necessary to control emission intensity to obtain a predetermined emission intensity reliably. This emission intensity control is called Automatic Power Control (APC). In the APC process, before the semiconductor laser diode is actually driven, the laser diode is driven in advance, quantity of light output from the laser diode is received by a photo diode (PD), and the detection current values of corresponding quantities of light are stored in a storage device. Then, the laser diode is controlled using the current values saved in the storage device so as to obtain a desired emission intensity reliably.
High resolution and high-speed operation are required of contemporary printers and copiers, including those that employ a semiconductor laser as a light source. In a case in which there is only a single laser beam used, in order to improve image resolution and printing speed it is necessary to increase modulation speed, which is the speed at which the semiconductor laser is driven (turned on and off) in accordance with the input image data. However, there is a limit to the modulation speed. Accordingly, in order to improve image resolution and operating speed without increasing the modulation speed, there is no alternative but to increase the number of laser beams.
In a case in which four laser beams are used, when it is assumed that the modulation speed and the printing speed are to the same as in a case in which a single beam is set in the laser light, the image resolution in the main-scanning direction and sub-scanning direction (horizontal and vertical directions) can be doubled. Alternately, in this case, when the image resolution is the same as a case in which a single beam is set in the laser light, the printing speed can be quadrupled.
As for the semiconductor laser used for the light source, edge-emitting laser elements (hereinafter “edge-emitting lasers”) that emit laser light parallel to the activation layer are widely used. When the edge-emitting laser is used, the number of beams is set to a single laser beam, or two or four multi-beam lasers are used in the printers and copiers. Since an optical axis between the lasers of the multi-beam laser is stable, when the multiple beams are required, it facilitates adjustment of the optical axis between the adjacent lasers by using the multi-beam laser rather than by adjusting multiple individual single lasers in the apparatus separately.
Generally, a laser unit of the edge-emitting laser includes a single photo-receiver element in addition to a multi-beam laser. The edge-emitting laser emits backward (back projection) proportional to a front projecting power as used as the laser light, and the photo-receiver element PD installed in the laser unit receives the back projection and generates a monitor current similarly proportional to the quantity of light received. In the multi-beam laser, even when the powers of front projection for respective laser diodes are identical, the monitor currents thus generated are slightly different among the beams due to individual variability.
FIG. 1 is a conceptual diagram of a related art laser (LD) board 700 mounting a related art laser driver 7 and a main board 60X mounting an image control unit 6, provided in an image forming apparatus. In the image forming apparatus, the optical unit, such as, the semiconductor laser driver 7, a laser unit, a polygon mirror, and the scanning lenses are configured as a laser scan unit (hereinafter “LSU”). In order to activate the semiconductor laser LD rapidly, the semiconductor laser driver 7 and the laser unit 1 are provided in a same LD board 7000, and disposed close to each other. It is preferable that the LD board is small so as to dispose the gap among the LSU. By contrast, the image control unit 6 is installed in a main board 60X including a central processing unit (CPU), random access memory (RAM), read only memory (ROM), and an image memory.
Herein, the semiconductor laser driver 7 mounted in the LD board 700 and the image control unit 6 mounted in the main board 60X are connected via a cable that is usually longer than 1 m. A supply voltage from a power supply and a ground voltage (GND) are supplied from the main board 60X to the LD board 700 via the cable. Because a consumption current of the semiconductor laser driver 7 and a driving current for emitting the semiconductor laser LD are transmitted through voltage-transmission lines connecting to the power supply or the ground voltage, voltage drop and voltage boost are generated in the voltage transmission lines by resistance of the cable.
Thus, voltage generated in the image control unit 6 differs from the voltage received in the semiconductor laser driver 7 in direct current (DC). In addition, the current fluctuates due to switching the laser diode LD on and off during image formation, and accordingly the supply voltage and the ground voltage in the LD board 700 fluctuate from point of an alternative current (AC). Thus, since the voltage fluctuates in the cable therebetween from the point of DC and AC, error may be generated in the emission amount (emission intensity) with respect to a setting value of the quantity of light.
In addition, of late there is market demand for a semiconductor laser driver that is capable of controlling the light-emission amount of the semiconductor laser with ever more precise timing, in order to cut cost and improve image quality.